mfd: axp20x: Add a cell for the ac power_supply part of the axp20x PMICs

As a counterpart to the usb power_supply cell, this commit adds an AC
power_supply cell to the axp20x driver.

Still missing are the RTC backup battery and the main battery charger
cells.

Signed-off-by: Michael Haas <haas@computerlinguist.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>